This invention relates to a chain tensioner used for maintaining tension of a timing chain that drives a camshaft of a vehicle engine.
Generally in an automobile engine, rotation of a crankshaft is transmitted to a camshaft through a timing chain, thereby opening and closing valves of combustion chambers by rotation of the camshaft. In many cases, to keep the tension of the chain within appropriate range, a tension adjustment device is used which comprises a chain guide provided pivotably around a support shaft and a chain tensioner pushing the chain guide toward the chain.
As a chain tensioner incorporated in this tension adjustment device, a chain tensioner is known comprising a cylinder having open and closed ends, a plunger axially slidably inserted in the cylinder, and a return spring biasing the plunger in the protruding direction from the cylinder. The plunger has an open end inserted in the cylinder and a closed end and is formed with an internal thread on the inner circumference. This chain tensioner further includes a screw rod having an external thread that is in threaded engagement with the internal thread of the plunger. The portion of the screw rod protruding from the plunger abuts a rod seat provided in the cylinder (See JP Patent Publications JP 10-132039A and JP 2001-124159A).
Also in this chain tensioner, a spring receiving hole is formed in the screw rod, receiving one end of the return spring and thereby reducing the axial length of the chain tensioner.
In the cylinder, an oil supply passage is formed, communicating with the pressure chamber defined by the plunger and the cylinder. The oil supply passage communicates with a through hole formed in the rod seat and this through hole communicates with the spring receiving hole through an oil path hole formed in the screw rod. In the through hole of the rod seat, a check valve is mounted, preventing reverse flow of hydraulic oil.
In this chain tensioner, when the tension of the chain increases during operation of the engine, the plunger is moved in the direction to be pushed into the cylinder (the “pushed-in direction”, hereafter) by the tension of the chain, and thereby absorbs the tension of the chain. At this time, the screw rod rotates relative to the plunger, being moved forward and backward within the gap of the internal and external threads by vibration of the chain. Also, hydraulic oil in the pressure chamber flows out through a leak gap between the sliding surfaces of the plunger and the cylinder. Viscous resistance of hydraulic oil makes the plunger move slowly with its dampening effect.
When the tension of the chain decreases during operation of the engine, the plunger is moved in the direction to protrude from the cylinder (the “protruding direction”, hereafter) by biasing force of the return spring, thereby absorbing looseness of the chain. At this time, the check valve opens and hydraulic oil flows into the pressure chamber through the oil supply passage, moving the plunger quickly.
Also, even if the tension of the chain increases because of the stop position of the cams when stopping the engine, since the chain does not vibrate while the engine is not running, the internal thread of the plunger is received by the external thread of the screw rod, thereby fixing the position of the plunger. This reduces loosening of the chain when restarting the engine, thereby making restart of the engine smooth.
The screw rod of the above-mentioned chain tensioner is shown in FIG. 8. This screw rod 41 is formed from a columnar blank. An external thread 42 on the outer circumference of the screw rod 41 is formed by rolling the outer circumference of the blank, and then an axial end of the blank is machined so as to form a contact surface 43 that contacts a rod seat (not shown) of the screw rod 41. The spring receiving hole 44 and the oil path hole 45 are also formed by machining after forming the external thread 42 by rolling.
However, it increases the processing cycle time and the cost to form the contact surface 43, spring receiving hole 44 and oil path hole 45 by machining.
To omit this machining, the contact surface 43 can be forged before rolling the external thread 42. That is, the contact surface 43 can be formed by forging one end of the blank before rolling the outer circumference of the blank 42 to form the external thread 42. However, in this method, the contact surface 43 may be deformed due to a flow of material during rolling of the external thread 42.
The object of this invention is to reduce the processing cycle time of a screw rod and a plunger, thereby reducing the production cost of a chain tensioner.